In wireless telephone and similar transceiver circuitry, a direct-conversion receiver can provide advantages over a superheterodyne receiver, including lower power consumption due to the absence of an intermediate frequency mixer, and smaller integrated circuit die area consumption due to less complex circuitry. However, conventional direct-conversion receiver architectures can suffer from poor sensitivity due to second-order distortion. For example, in Wideband Code Division Multiple Access (WCDMA) transceivers, in which a transmit signal is present while the receiver is receiving information, second order distortion in the receiver causes second-order intermodulation products of the transmit signal to fall coincident with signal to be received. As the transmit signal can be 60-80 dB larger than the signal to be received, features must be included in the receiver to enhance second-order performance to provide sufficient sensitivity to receive the signal. Second-order intermodulation distortion is typically expressed in terms of a second-order intercept point (IP2) value, in units of dBm.
In a conventional direct conversion receiver, a signal received at the antenna is downconverted by two mixers that combine the received signal received with in-phase (I) and quadrature (Q) signals produced by a local oscillator. It has been proposed to optimize the IP2 in such a receiver by adjusting the mixer parameters. Each mixer is generally implemented as a transconductor comprising a differential pair of transistors plus a “switching quad” comprising four transistors. It has been proposed to optimize receiver IP2 by adjusting the direct current (DC) at the switching quad. However, such a method can degrade the I/Q balance of the receiver and degrade signal-to-noise ratio. It has also been proposed to optimize receiver IP2 by tuning the load resistors in the mixer quad to match the positive and negative differential path gains and thereby eliminate or compensate for second-order intermodulation products. This approach may work well enough in a voltage-mode mixer, but in a current-mode mixer it can cause a large DC offset that adversely affects baseband circuitry operation.